FIG. 1 schematically shows an optical modulator according to the Mach-Zehnder interferometer principle, commonly referred to as an MZI modulator. The modulator includes an optical waveguide receiving a power Pin, which is divided into two branches 12a and 12b at a point S. The two branches come together again at a point J. Each branch carries half of the original optical power.
Each branch comprises a static electro-optical phase shifter SPS (SPSa and SPSb) and a dynamic electro-optical phase shifter DPS (DPSa and DPSb). The static phase shifters SPS are used to define an initial phase difference φ0 between the two optical waveguide branches. They are controlled by respective bias signals IBa and IBb. The dynamic phase shifters DPS are used to perform a differential modulation around the initial conditions defined by the SPS phase shifters. They are controlled by respective modulation signals M and M/ varying in phase opposition.
The waves arriving on both branches of the modulator are added at point J. The resulting wave has a power of Pin·cos 2(Δφ/2), neglecting the optical losses, where Δφ is the instantaneous phase difference between the waves of the two branches.
FIG. 2 is a perspective view of the waveguide branches 12a and 12b incorporating phase shifters SPS and DPS, shown in gray. As shown, the waveguides are formed in transparent islands of intrinsic semiconductor material, having an inverted “T” cross section with a central rib WG that conveys the optical beam. The phase shifters are configured to replace waveguide segments and have the same inverted “T” cross section. The edges of the phase-shifters bear electrical contacts for controlling the phase-shifters—the edges generally rise above the plane of the waveguides, as shown, to reach the device metal levels.
FIG. 3A is a schematic sectional view of a DPS phase shifter referred to as a High-Speed Phase Modulator (HSPM). The cross section plane is perpendicular to the axis of the optical waveguide. A dashed circle, at the center of rib WG, represents the area of concentration of the optical beam.
The phase shifter comprises a semiconductor structure of the same nature as that of the waveguide, generally silicon, forming a PN junction 14 in a plane parallel to the axis of the waveguide, and offset with respect to the axis. The junction 14 is shown, as an example, at the right lateral face of the rib WG.
To the left of the junction 14 extends a P-doped zone that has a cross section conforming to the cross section of the waveguide, comprising an elevated portion at the level of the rib WG, and a lower lateral wing toward the left edge. Zone P ends at its left by a P+ doped raised area, bearing an anode contact A.
To the right of junction 14 extends an N-doped wing conforming to the cross section of the waveguide. The wing ends to the right by an N+ doped raised area, bearing a cathode contact C. The structure of the phase shifter may be formed on an insulating substrate, for example a buried oxide layer BOX.
To control the phase shifter of FIG. 3, a voltage is applied between the anode and cathode contacts A, C, which reverse-biases the junction 14 (the “plus” on the cathode and the “minus” on the anode). This configuration causes a displacement of electrons e from the N region to the cathode and of holes h from the P region to the anode, and the creation of a depletion region D in the vicinity of the junction 14. By adjusting the amplitude of the bias voltage, the carrier concentration can thus be modified in the zone WG crossed by the optical beam, which results in a corresponding modification of the refractive index of this zone.
FIG. 3B is a schematic cross section view of a PIN junction phase shifter SPS. The central N and P-doped zones of the structure of FIG. 3A are replaced by a single intrinsic semiconductor zone I, in practice a zone having a floor P-doping level. To control this phase shifter, a current is applied between the anode and cathode contacts A, C, which forward-biases the junction (the “minus” on the cathode and the “plus” on the anode). A current establishes between the anode and the cathode causing the injection of carriers in the intrinsic zone I (holes h from the P+ region to zone I and electrons e from the N+ region to zone I). The carrier concentration, i.e. the refractive index, is thus changed as a function of the current in the area crossed by the optical beam.
PIN phase shifters provide a larger adjustment range per unit of length than HSPM phase shifters, but their response speed is 50 to 100 times slower; that is why they are used in static mode to adjust the quiescent conditions of the modulator. HSPM shifters offer a small response range per unit of length. In practice a PIN phase shifter may introduce a phase delay of 90° over only 250 microns, while an HSPM phase shifter provides a phase shift amplitude of about ten degrees per millimeter. If a phase shift amplitude of 30° is desired, the HSPM phase shifter spans nearly 3 mm. FIG. 2 thus illustrates the HSPM (DPS) phase shifters as being longer than the PIN phase shifters.